In conventional optical instruments, the size of the exit pupil is determined by a function of the numerical aperture and overall magnification of the optical instrument, and hence the size of the exit pupil is of fixed and relatively-small dimension. Consequently, it is necessary for an observer to accurately align the entrance pupil of his/her eye with the exit pupil of the optical instrument in order properly to view an image.
The present applicant has previously developed a number of different optical instruments which, through the provision of a diffractive element at an intermediate image plane, provide an exit pupil which is effectively enlarged, allowing an observer to view an image by placing his/her eye anywhere within the enlarged exit pupil. These optical instruments are disclosed in U.S. Pat. No. 6,028,704, U.S. Pat. No. 6,608,720, U.S. Pat. No. 7,123,415 and GB-A-2360604. Another optical instrument which incorporates an exit pupil expander is disclosed in US-A-2005/0237615.
The present applicant has now developed improved optical instruments, which still provide an exit pupil which is effectively enlarged, but which can provide for greater optical clarity and also avoid the need for a field lens arrangement, which, given the size of the field of view, usually represents the most expensive component within the optical instruments, being a precision-ground and polished glass component.